1. Field of the Invention
The present invention relates to an optical information processing apparatus for recording and reproducing information on an information storage medium capable of optical recording and reproduction of information, e.g., optical disks or optical cards.
2. Description of the Related Art
In the field of optical memory technologies utilizing optical disks or optical cards, there is an increasing variety of applications including digital audio disks, video disks, data files, and the like. According to optical memory technologies, the information on an information storage medium can be reproduced by using a fine beam of light to read minute convex and concave portions, or marks and spaces (which have distinguishable physical properties from each other) provided on the medium.
FIG. 22 is a diagram illustrating the relationship between aforementioned marks and a fine light beam, where information is recorded based on differential orientations of magnetization on an optical disk as an information storage medium. Hatched regions 302 are marks (A), and blank regions 303 are marks (B), such that marks (A) and marks (B) have different magnetization orientations. A series of repetition of marks (A) 302 and marks (B) 303 are defined as a track. Information can be reproduced as a converged light beam 306 travels over a track.
The size of the converged light beam 306 is a function of the numerical aperture (NA) and wavelength (.lambda.) of the optical system. If a mark length (defined as the length of mark (A) 302 or mark (B) 303 along the longitudinal direction of tracks) is too small relative to the size of the light beam 306, it becomes difficult to obtain enough modulation of a reproduction signal which is output from an optical head. Therefore, selecting a small minimum mark length for increased recording density results in an increased intersymbol interference (which is essentially the influence from preceding and subsequent marks), thereby making it difficult to maintain a low error rate in the reproduction of information.
Accordingly, a method (Japanese Laid-open Publication No. 6-290496; see ISOM '97 lecture proceedings (Tu.about.E-04)) has been proposed which virtually enlarges recorded marks to the size of alight beam before reproduction of information.
However, such conventional methods have a problem in that, during reproduction of information recorded on an information storage medium, the generation of specific signal waveforms causes an increase in the error rate in the signal reproduction by a signal reproduction circuit.